Method for making lightweight proppant for oil and gas wells

ABSTRACT

A lightweight oil and gas well proppant made by simultaneously mixing and compacting a mixture of kaolin clay which has been calcined at a temperature low enough to prevent the formation of mullite and crystobalities to an LOI of 12 or less when tested at 1400° C., and amorphous to microcrystalline silica both of which have been milled to an average agglomerated particle size of 7 microns or less to form green pellets, and then drying, screening and sintering the pellets to form proppant pellets having a specific gravity of 2.7 or less, the proppant having a conductivity of at least 3,000 millidarci-feet as measured by the StimLab Technique after 50 hours at 8,000 psi and 275° F., in the presence of deoxygenated aqueous 2% solution of KCl using sandstone shims.

This is a divisional of copending application Ser. No. 07/297,876, filedon Jan. 17, 1989, now U.S. Pat. No. 4,921,820, which is acontinuation-in-part of Ser. No. 07/277,258, filed on Aug. 2, 1988, nowabandoned.

FIELD OF INVENTION

This invention relates generally to a method of making lightweightproppants for oil and gas wells and more particularly to a method ofmaking lightweight proppants for oil and gas wells which are lighter inweight than existing lightweight proppants but which have strength andconductivity similar to and preferably substantially higher than suchexisting lightweight proppants.

BACKGROUND

Hydraulic fracturing is a process of injecting fluids into a selectedoil or gas bearing subsurface earth formation traversed by a well boreat sufficiently high rates and pressures such that the formation failsin tension and fractures to accept the fluid. In order to hold thefracture open once the fracturing pressure is released a propping agent(proppant) is mixed with the fluid which is injected into the formation.

Hydraulic fracturing increases the flow of fluids from an oil or gasreservoir to a well bore in at least three ways: (1) the overallreservoir area in communication with the well bore is increased, (2) theproppant in the fracture generally has significantly higher permeabilitythan that of the formation, thereby allowing fluids to flow more easily,and (3) the high conductivity channel causes large pressure gradients tobe created in the reservoir past the tip of the fracture.

Proppants are generally strong, preferably substantially spherical,particulates that should be able to withstand the high temperatures andpressures and corrosive environments experienced in the subsurfaceformations surrounding an oil or gas well. Early proppants were formedof material such as glass beads, sand, walnut shells and aluminumpellets. These materials did not have sufficient strength or resistanceto corrosion to be successful in many wells, particularly where closurepressures above a few thousand p.s.i. were experienced.

U.S. Pat. No. 4,068,718 to Cooke relates to a proppant which Cookestates is formed of "sintered bauxite" that has a specific gravitygreater than 3.4. Cooke states that specific gravities above 3.4 arerequired in order that the proppant have sufficient compressive strengthto resist fragmentation under the high stress levels experienced in use.While the proppant described in Cooke's example proved to havesufficient strength to resist crushing, the high specific gravity wasundesirable since it required the use of higher viscosity fracturingfluids and resulted in a lower volumetric proppant concentration for agiven weight of proppant loading in a fracturing fluid when comparedwith that achieved by a proppant of lower specific gravity. In general,the higher the volumetric concentration of the proppant in thefracturing fluid, the wider the propped fracture will be after thefracturing pressure is released.

U.S. Pat. No. 4,427,068 to Fitzgibbon relates to intermediate strengthcomposite proppants made by mixing calcined diaspore clay, burley clayor flint clay with alumina, "bauxite" or mixtures thereof such that theratio of alumina to silica in the composite mix is between nine to oneand one to one. The powdered starting materials are mixed in an Eirichmixer and while the mixing is in progress sufficient water is added tocause formation of composite spherical pellets from the powderedmixture. Fitzgibbon states that the rate of water addition is notcritical. The pellets are dried and then furnaced to sinter the pellets.The sintered pellets have a specific gravity of between 2.7 and 3.4.

U.S. Pat. No. 4,522,731 to Lunghofer relates to an intermediate strengthproppant having an alumina content between 40% and 60% which is producedusing a spray agglomeration process and which has a density of less than3.0 gr/cc. In a preferred embodiment Lunghofer produces his proppantsfrom "Eufaula bauxite" which it states is bauxitic-kaolin type materialdeposited in and around Eufaula, Ala. According to Lunghofer, theEufaula bauxite preferably contains at least some (above 5%) gibbsite.

U.S. Pat. No. 4,668,645 to Khaund relates to an intermediate strengthproppant made from a mined "bauxitic clay" having a specified chemicalcomposition.

The proppants described in the Fitzgibbons, Lunghofer and Khaund patentshave specific gravities lower than that of the earlier Cooke proppantand proppants having such lower specific gravities have been used withsome success in intermediate depth wells where the stress on theproppant is in the 5,000 to 10,000 p.s.i. range. It will be desirable,however, to have still lighter weight proppants which are easier totransport in the fracturing fluid and are therefore carried farther intothe fracture before settling out and which will yield a wider proppedfracture than the known lower specific gravity proppants. The lighterweight proppant should, however, have a conductivity rating at least ashigh as and preferably substantially higher than those obtainable withthe presently available "lightweight" proppants.

The conductivity of a proppant under specific conditions of stress,temperature, corrosive environment and time is the single most importantmeasure of its quality. The conductivity of a packed proppant such asmight be deposited in a fracture is defined as the permeability of theproppant pack multiplied by the width of the propped fracture and isusually stated in units of millidarci-feet ("md-ft").

The conductivity of currently available intermediate strength proppantsis frequently measured by the tentative API 8 hour procedure, "TentativeFifth Draft of Recommended Prac- tices For Evaluating Short TermProppant Pack Conductivity", (March 1987) (hereinafter the "API 8 hourProcedure"), which procedure is hereby incorporated by reference.

Recently a consortium of some twenty-eight organizations involved invarious aspects of the fracturing and stimulation business has sponsoredresearch on ways of evaluating and improving stimulation techniques.Stim-Lab, Inc. of Duncan, Okla. acts as the testing arm of theconsortium to develop consistent and repeatable testing procedures forproppants including tests for determining their permeability andconductivity. The long term conductivity testing techniques developed byStim-Lab have been widely accepted in the industry and are described ina publication of the Society of Petroleum Engineers, No. SPE 16900,entitled "An Evaluation of the Effects of Environmental Conditions andFracturing Fluids on the Long-Term Conductivity of Proppants" by G.S.Penny of Stim-Lab, Inc., which publication is hereby incorporated byreference. It should be understood that any gap in the description inthe SPE publication should be filled in by reference to the API 8 hourProcedure. The testing techniques used by the applicants to determinethe conductivity of the proppants of the present invention as they areintended to be supplied to a customer (referred to as the "Stim-LabTechnique") are essentially identical to those described in SPEpublication No. 16900 using Monel-K 500 or sandstone shims in theconductivity cells, as noted herein A single cell was used rather thanstacking 4 cells in the manner described in the SPE publication. Thishowever should have no effect on the measured results. The Stim-LabTechnique is considered to yield conductivity measurements that arerepeatable to within about 5-10%.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a low density, highstrength proppant which is formed of solid ceramic particles having adry specific gravity less than 2.70 and preferably 2.60 or less and aconductivity of at least about 3,000 md-ft and preferably at least 4000md-ft after 50 hours at 8,000 p.s.i. and 275° F. in the presence of adeoxygenated 2% aqueous solution of KCl as measured by the Stim-LabTechnique using sandstone shims. Most preferably the conductivity is atleast 4400 md-ft as measured under the above conditions.

The method of the present invention involves producing a proppant bymilling calcined kaolin clay and an amorphous to microcrystalline silicato an average particle size of less that 7 microns, and preferably about3.0 microns or less and pelletizing a mixture of the resulting powdersin a compacting mixer. Preferably the mixture contains between 55 weightpercent ("w/o") and 90 w/o kaolinite and between 45 w/o and 10 w/osilica, although the percentages may fall outside these ranges.Kaolinite is normally the major component of the mixture. However, inthe limit the silica content could reach 100% yielding a proppant ofessentially all crystobablite having a specific gravity of about 2.33.The kaolinite and silica are preferably milled together

The kaolin clay is calcined at a temperature of less than 900° C. toreduce the loss on ignition ("LOI") to 12 W/o or less when tested at1400° C., and preferably to about 2 w/o LOI when tested at 1400° C. Itis important that the calcining be done at a low enough temperature thatthe kaolin clay does not undergo a phase transformation to mullite andcrystobalite. The silica should not contain any significant amount ofcrystalline quartz, other than amorphous to microcrystalline quartz ashereinafter defined, and is preferably dried to a LOI of about 1 w/owhen tested at 1000° C.

Preferably, an organic binder is first dispersed in the milled materialin the mixer and then water is added to the powder at a controlled ratewhile mixing and compacting the powder to form rough pellets of adesired size. Next, with the mixer still running, additional dry, milledmaterial is slowly added to yield smooth, spherical pellets. The pelletsare then dried and fired to convert the material to proppant pelletsthat preferably comprise between about 35 (w/o) and 60 w/o mullite,between about 35 w/o and 60 w/o crystobalite and a minor amount (lessthan 10 w/o) of a glassy phase. The proppant has a specific gravity ofless than 2.70 and preferably of 2.60 or less. Most preferably thepellets comprise between 35 w/o and 50 w/o mullite and 50 w/o to 60 w/ocrystobalite.

The proppant produced in accordance with the method of the presentinvention is mixed with a fluid and injected into a subterraneanformation under high pressure to open a fracture, with the proppantremaining in the fracture to prop it open after the fracturing pressureis removed. The measured conductivity of the proppants of the presentinvention are equal to and usually substantially better than those ofother known lightweight proppants which have higher specific gravitiesand higher weight per unit of volume.

DETAILED DESCRIPTION

The preferred raw materials for use in making the low specific gravity,high strength proppants of the present invention are kaolin clay andamorphous to microcrystalline silica. Preferably the kaolin clayconsists largely of kaolinite (Al₂ Si₂ ₅ (OH)₄) and is essentially freeof sand (i.e., quartz). One source of such material is from C E.Minerals, headquartered in King of Prussia, Pa. The kaolinite depositsowned by C. E. Minerals are mined at C. E. Mineral's Mulcoa operationsin Andersonville, Ga. The Andersonville kaolin deposits owned by C. E.Minerals are well described in a report by Alfred D. Zapp entitled"Bauxite Deposits of the Andersonville District, Ga", U.S. GeologicalSurvey Bulletin 1199-G. This report is incorporated herein by reference.The report states that the deposit consists largely of the mineralkaolinite and is essentially sand free. The chief impurities arecompounds of iron and titanium and small amounts of gibbsite. The kaolinclay is deposited in tabular lenticular masses. Raw materials containingsignificant amounts of sand (free silica or quartz) produce a weakerproppant, while raw materials containing gibbsite or other hydratedaluminas yield a product having an undesirably high specific gravity.

By amorphous to microcrystalline silica is meant silica which is trulyamorphous or is "amorphous" in the sense that the ultimate submicronparticles, as seen with the aid of a scanning electron microscope, donot have the angular shape of crystals. One source of amorphous tomicrocrystalline silica is from Illinois Minerals Company in Cairo, Ill.This silica, as described by Illinois Minerals, is an extremelyfine-grained microcrystalline silica formed by weathering of silica-richlimestone. Materials from other deposits of kaolinite and of amorphousto microcrystalline silica can also be used in making the proppants ofthe present invention.

In general the kaolin clay most useful as a raw material in making theproppants in accordance with the invention may contain by chemicalanalysis about 45 w/o alumina and 52 w/o silica, less than 1 w/o ironoxide and less than 2 w/o (preferably less than 1 w/o) free quartz.Preferably the kaoline clay approaches 100% kaolinite. Most preferablythe amount of free quartz is non-detectable.

Other naturally occurring minerals which may be present in minor ortrace amounts include anatase and rutile. Minerals whose presence in theraw material appear to be somewhat detrimental to the properties of thefinal product include quartz, pyrite, marcasite, siderite, micas andmontmorillonites. The amorphous to microcystalline silica most useful asa raw material in making the proppants in accordance with the inventionshould contain by chemical analysis at least 94 w/o (preferably 100 w/o)silica.

The kaolin clay is preferably calcined before further processing inorder to remove water and organics. In accordance with the invention,applicants have found that the calcining should be performed at atemperature low enough that the kaolin clay does not undergo a phasechange to form mullite or crystobalite. Applicants have found that thepresence of any significant amount of mullite or crystobalite in thekaolin clay before the final drying and firing steps has a severelydetrimental effect on the properties of the proppants produced.Therefore the calcining should be performed at a temperature below 900°C. for a sufficient time that the loss on ignition ("LOI") of thekaolinite is 12 w/o or less and preferably 2 w/o or less when tested at1400° C. The calcining may be performed relatively quickly attemperatures on the order of 700° to 800° C. or may be performed moreslowly at lower temperatures. If the calcining temperature is aboveabout 450° to 500° C., the kaolinite is converted to amorphous aluminaand silica (sometimes referred to as "meta kaolinite"). Such atransformation, however, has no adverse effect on the product of theinvention. The silica should be dried after mining to an LOI of lessthan 1 w/o when tested at 1,000° C. This drying is typically done attemperatures of between about 150° and 200° C.

The calcined material and the silica is then reduced in particle size,preferably by dry ball milling them in a closed loop system containing aparticle classifier. The average agglomerated particle size in thismilled material is less than 7 microns and preferably about 3.0 micronsor less as measured by a Sedigraph (Micro Meritics Instrument Corp.) ora Granulometre' (Cilas Compagnie Industrielle des Lasars). The trueultimate particle size, however, of the milled raw material (both thekaolin and silica) is much finer than 3.0 microns and is believed to bemade up of submicron flakes or particles. Such submicron flakes orparticles tend to agglomerate to form the composite particles which aresensed by the measuring instruments. The calcined kaolin clay and silicaare mixed together and preferable are milled together. The mixturepreferably contains between 55 w/o and 90 w/o kaolinite and between 45w/o and 10 w/o silica, although different percentage mixtures may beused. Kaolinite is normally the major component of the mixture. However,in the limit the amorphous to microcrystalline silica content can reach100%, yielding a proppant that is essentially entirely crystobalitehaving a specific gravity of about 2.33. Such a super lightweightproppant would be very useful in shallow wells when closure stresses areon the order of 4000 p.s.i. or less.

The milled raw material is pelletized preferably in a compacting mixer.The preferred commercially available machines for this purpose are theEirich Countercurrent Intensive Mixers which are manufactured in severalsizes of different capacities by the Maschinenfabrik Gustav Eirich ofHardheim, West Germany and distributed in the United States by EirichMachines, Inc., New York, N.Y.

The Eirich mixer has a rotating mixing pan forming the bottom of themixing chamber which pan can be either horizontal or inclined at anangle and a "mixing star" which rotates in the opposite direction fromthe pan. The mixing star rotates about an axis parallel to and offsetfrom that of the pan and has a diameter of about 20 to 25 percent ofthat of the pan. The preferred form of mixing star for the pelletizingoperation is referred to by Eirich as a suction type rotor and has aplurality of vertically extending trapidzoidally shaped mixing elementswhich extend from the periphery of the rotating element. For the ModelR7 machine, the pan has a single rotational speed of about 45 r.p.m.,while the mixing star has a low and a high speed. The low speed is about700 r.p.m. and the high speed is about twice that, about 1400 r.p.m. Therotational speeds of the elements of the larger mixers are adjusted sothat rotating elements have similar tip speeds. The mixer may alsocontain a second mixing star upon which can be mounted plows forcleaning the sides of the rotating pan. This second mixing star is alsomounted eccentrically in the mixer and rotates in the opposite directionfrom the mixing pan. An example of another compacting mixer that isbelieved suitable for use in the present invention is the LittlefordLodige mixer.

The milled powder charge is preferably first mixed dry in the mixer withup to about 3 w/o (calculated on the basis of the initial milled powdercharge) of an organic binder such as gelatinized cornstarch for a periodof about 30 seconds with the suction rotor on the low speed setting inorder to fully disperse the binder in the raw material powder.

It is preferred that such binders be such that they are pyrolized ordriven off during later firing of the proppant pellets. The amount ofthe binder is normally on the order of 1 w/o to 3 w/o of the initialmilled powder charge. It is possible to make pellets with the method ofthe invention without a binder but it is believed that the use of abinder results in improved properties and yield.

Next water is added at a controlled rate to the mixer with the suctionrotor running at the low speed setting to initially pelletize thematerial. The rate of water addition should be controlled properly tofacilitate the formation of the pellets in the mixer and can affect thequality of the final product. Typically the total amount of water forthe Model R7 Eirich mixer is from about 35 to 55 w/o of the initialstarting material and preferably about 43 to 50 w/o. Preferably betweenabout 40 and 60 w/o of the expected total amount of water is added afterthe binder is dispersed and mixed for about 2 to 4 minutes. Thereafteradditions of about 8 to 10% of the total water are made about every 0.75to 1.5 minutes until irregularly shaped pellets ranging in size fromabout 1/16 to 1/8 inch in diameter are formed.

Once pellets have formed in the mixer, which at this point are normallylarge and irregularly shaped, the suction rotor is switched to the highspeed setting and additional milled calcined raw material (referred toas "dust") is slowly added to the mixer over a period of about 5 to 7minutes in order to cause the pellets to become smaller, well shapedspheres of a narrower size distribution, typically 0.02 inches to 0.047inches. Typically, the amount of dust required to form the desired sizespheres is about 42 to 70 w/o of the starting raw material. Preferablyabout 46 to 54 w/o dust is added.

In some cases the suction rotor is switched back to the low speedsetting for a later portion of the 5 to 7 minute period after a majorportion of the dust has been added to the mixer. After adding the dust,the suction rotor is then preferably set back to the low speed settingif it has not already been and the mixer run for a further period ofabout 30 seconds in order to improve the sphericity of the pellets andto increase yield of the desired size range.

The amount and rate of water and dust additions to the mixer can have asignificant impact on the conductivity of the proppant produced by themethod of the invention. If too much water is added, the particles arepoorly shaped and are too large. If the dust is added too quickly itresults in a poor yield of particles in the desired size range. If notenough dust is added the particles are poorly shaped and will sticktogether in agglomerated masses. If too much dust is added the particleshave a rough surface and unconsolidated dust remains in the mixer. Whenthe right amount of dust is added, the process yields smooth, spherical,well compacted pellets. The spherical pellets are next discharged fromthe mixer and dried to a free moisture content of less than 10% andpreferably less than 5%. The drying temperature and time does not appearto be critical and it has been found, for instance, that drying attemperatures of 150° C. overnight, or of 260° C. for 10 to 15 minutesare suitable. Once dried, the desired size spherical pellets areseparated by screening for instance to a 18/35 mesh size. The largerthan 18 mesh and finer than 35 mesh are recycled to be repelletized. Thescreened particles in the desired size range are then sintered, forinstance in a rotary kiln, at a temperature between 1300° and 1500° C.,preferably between 1375° to 1425° C. for about 15 to 30 minutes attemperature. The total time in the kiln is normally from about 1 to 11/2hours. The particles shrink upon firing by about 15% to 20%. The desiredfinished size is typically between about 0.0167 inches and 0.0331 inchesbut can be larger or smaller as needed. The particular temperature towhich the screened particles are fired is selected in order to maximizetheir strength. This temperature depends on several variables, some ofwhich are the raw material mineralogy, the milled particle size, thepellet size and the volume of material in the kiln.

After firing, the pellets are again screened to the desired final size.A typical product size is 20/40 mesh which contains 90 w/o of itspellets of between 0.0167 inches and 0.0331 inches in size andpreferably 90 w/o of the pellets between about 0.0232 inches and 0.0331inches in size.

The proppants of the invention have been found to have surprisingly andunexpectedly high conductivities for having such low specific gravities.More particularly, the conductivities of the proppants of the preferredembodiment of the present invention are at least 3000 md-ft andpreferably at least 4000 md-ft after 50 hours at 8000 p.s.i. and 275° F.in the presence of a deoxygenated 2% aqueous solution of KCl as measuredby the Stim-Lab Technique using sandstone shims. Most preferably theconductivity is at least 4500 md-ft when measured under the aboveconditions. The proppants of the present invention have specificgravities of less than 2.70, and preferably 2.60 or less, which arelower than those of other commercially available lightweight proppants.In fact, the preferred proppants of the present invention are evenlighter than sand (specific gravity 2.64), the lightest proppant whichhas heretofore been in common commercial use. The conductivity of theproppants of the present invention are between about 3 and 20 times thatof sand, however, depending upon operating conditions. Sand is of littleuse at pressures on the order of 8000 p.s.i. while the preferredproppants of the present invention still have high conductivities, onthe order of as much as 4500 md-ft, at such pressures.

Compared to another heavier commercially available proppant which isconsidered to be in the lightweight range, the preferred proppants ofthe present invention have substantially higher conductivities. Thecommercially available "lightweight" proppant has been measured inaccordance with the Stim-Lab technique to have a conductivity of 6067md-ft after 50 hours at 250° F. and 6000 p.s.i. closure stress in thepresence of deoxygenated aqueous 2% KCL solution using sandstone shims.A proppant in accordance with the preferred embodiment of the presentinvention had conductivity values of 7855 md-ft when measured under thesame conditions. The same commercially available "lightweight" proppanthad conductivities measured by the Stim-Lab technique of 3616 to 3700md-ft after 50 hours at 275° F. and 8000 p.s.i. while the proppant ofthe present invention had a conductivity of 4459 md-ft under similarconditions.

The Loose Pack Bulk Density (LPBD) of the proppant of the invention usedin the above example is 1.45 gms/cm³. The LPBD of the commerciallyavailable "lightweight" proppant used for comparison in the aboveexample is 1.61 gm/cm³. Their specific gravities were 2.56 and 2.70respectively.

From a mineralogical point of view the sintered pellets of the presentinvention are preferably between about 35 w/o and 60 w/o mullite andbetween about 38 w/o and 60 w/o crystobalite and a minor amount (lessthan 10 w/o) of a glassy phase. It is believed that the glassy phase isbest minimized for the highest quality product.

EXAMPLES

The invention is further illustrated by reference to the followingnon-limiting examples wherein all percentages are by weight unlessotherwise specified. When applicable calcining conditions and millingtechniques for each raw material are given with each example. Unlessotherwise indicated, the pellets are screened to an 18/35 mesh sizeafter drying and to a 20/30 mesh size after sintering wherein 90% of thepellets are between about 0.0232 inches and 0.0331 inches in size. Thetypical yield of properly size pellets in the first screening is about70%. The chemical compositions of the raw materials used in thefollowing examples are summarized in Table I. The mineralogicalcomposition of the calcined kaolin clays as measured by the X-raydiffraction technique is summarized in Table II. The kaolinite in boththe Huber 40-C clay and the Mulcoa clay have been transformed toamorphous alumina and silica by being calcined at 750° C. and thus areno detected by X-ray diffraction. The figures in Tables II for the ACCOclay are for the uncalcined material so that kaolinite is detected.Mineralogically the silicas used in the examples is of such a fine grainsize as to be virtually undetectable as measured by X-ray diffractiontechniques. The components in Table II are divided into major mineralswhich comprise about 95 w/o of the detected minerals present and theminor minerals which together make up less then 5 w/o of the detectedminerals present. Kaolinite is approximately 45 w/o Al₂ O₃. Aftercalcining or drying all the raw materials are milled to an averageparticle size of about 3 microns as measured by a Granulometre. About90% of the particles were measured to be less than 10 microns in size.

                  TABLE I                                                         ______________________________________                                                  1          2        3       4                                                 Huber 40-C ACCO     Mulcoa                                          Example   Clay       Clay     Clay    (Silica)                                ______________________________________                                        Chemistry                                                                     Al.sub.2 O.sub.3                                                                        45.0       45.9     43.7    0.7                                     SiO.sub.2 53.0       51.4     51.8    99.0                                    TiO.sub.2  1.8        1.8      1.7    0.5                                     Fe.sub.2 OO.sub.3                                                                        0.2        0.9      0.8    0.5                                     ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Example          1         2        3                                         ______________________________________                                        Mineralogy                                                                     Major Minerals                                                               Kaolinite        ND        Major    ND                                        Minor Minerals                                                                Anatase          Major     Major    Trace                                     Rutile           ND        Trace    ND                                        Quartz           ND        ND       Trace                                     Mullite          ND        ND       ND                                        Amorphous        Yes       No       Yes                                       ______________________________________                                         "ND" means nondetectable                                                 

EXAMPLE I

One hundred (100) lbs. of a kaolin clay/silica mixture was produced bycombining 86 lbs. of calcined Huber 40-C kaolin clay (manufactured bythe J. M. Huber Corporation) and 14 lbs of dried Imsil A-108microcrystalline silica (manufactured by Illinois Minerals Company) inan Eirich Model R-7 Compacting Mixer and mixing the two ingredientstogether for 60 seconds. Both the calcined kaokin clay and the silicahad been milled separately to an average particle size of 3 microns asmeasured by a Granulometre. Hereafter in this example this mixture willbe referred to as "blended material".

A 50 lbs. charge of blended material was placed in an Eirich CompactingMixer having an inclined pan and a suction type rotor along with 11/2lbs. of gelatinized cornstarch binder and mixed for 30 seconds with thesuction rotor in its low speed setting in order to fully disperse thebinder in the blended material. With the suction rotor rotating at itslow speed setting, 9 lbs. of water was added to the mixer and mixed fora period of two minutes. As the suction rotor continued to operate inthe low speed setting, seven 2 lb. additions, followed by a 1 lb.addition and then a 1/2 lb. addition of water were added sequentially tothe mixer with a 45 second mixing period after each addition. At thispoint irregularly shaped pellets had formed.

The rotor was then switched to the high speed setting and an additional17 lbs. of the same blended material was slowly added to the mixer overa three minute period. Then the rotor was reset to its low speed settingand additional 11 lbs. of the blender material was added over a periodof two minutes. The mixing was continued with the suction rotor in thelow speed setting for an additional period of 30 seconds. At this pointthe particles had fairly good sphericity but had rather rough surfaces.An additional 7 lbs. of the blended material was slowly added to themixer over a 1 minute period and the material mixed for an additional 30seconds with the suction rotor continuing in the low speed setting. Thepellets now had a good spherical shape and smooth surfaces. The pelletswere discharged from the mixer and dried overnight at 150° C. in a boxoven. After drying the pellets were screened to an 18/35 mesh and firedat 1405° C. in a rotary kiln for a period of about 30 minutes attemperature with a total time in the kiln of approximately 1 hour 15minutes. After cooling the pellets were screened to a 20/30 mesh. Thedry specific gravity of the material was 2.56 as measured by a Beckmanair comparison pycnometer Model 930.

A conductivity test using the Stim-Lab Technique referred to above wasconducted with final closure stress of 6,000 p.s.i. at 250° F. in thepresence a deoxygenated aqueous 2% solution of KCl for 50 hours. Inaccordance with this procedure 63.06 grams of the screened proppantpellets were loaded into a API Hasteloy-C 10 in² linear flow cell togive a loading of 2 lbs./ft² of proppant and leveled loosely with auniversal bevel blade device. A 3/8 inch thick sandstone core was placedon top of the test pack followed by an O-ring fitted to a piston whichwas lightly coated with vacuum grease. The loaded test cell was thenplaced in a 150 ton Dake press and the closure stress was increased to500 p.s.i. at a rate of 100 p.s.i./min. The cell was saturated withdeoxygenated aqueous 2 w/o KCl solution and then purged of air at theambient laboratory temperature of 70° F. to 80° F. A Validyne DP15- 30differential pressure transducer connected across the cell wascalibrated with water columns to .0001 p.s.i. accuracy. Closure stresswas then raised to 1,000 p.s.i. at a rate of 100 p.s.i./min. A ReservoirAccumulator, made up of two 5 gal and two 1 gal nitrogen driven fluidreservoir accumulators which were filled with a 2 w/o KCl aqueoussolution that had been deoxygenated with nitrogen to a level of lessthan 15 ppb and preferably less than 5 ppb of oxygen was connected tothe test cell and set at a driving pressure of 400 p.s.i.. Theconnection of the Reservoir Accumulator to the cell is make through two150 ml sample cylinders filled with 100 mesh Oklahoma #1 sand withceramic band heaters in order to saturate the test fluid with silica. Itshould be noted that the closure stresses on the proppant pack in thecell are stated in terms of the net closure stress on the pack which isequal to the gross pressure applied by the press minus the 400 p.s.i.pressure applied by the reservoir accumulator.

The system was allowed 30 minutes to come to equilibrium and a series offive conductivity measurements were taken and averaged. The conductivitywas calculated from the darci relationship:

    K.sub.wf =26.78μQ/δP where

k_(wf) =Conductivity (md-ft)

26.78=factor to account for a 11/2×5 inch flow area

and pressure in p.s.i.

μ=Viscosity of flowing fluid at temperature (cp)

Q=Flow rare (ml/min)

δP=Pressure differential across 5 inch flow path.

After the readings were taken at ambient temperature, the temperaturewas increased to 250° F. and held for 8 hours for temperatureuniformity. Next readings with the system at 250° F. were taken at 1,000p.s.i., 2,000 p.s.i., 4,000 p.s.i. and 6,000 p.s.i. with the closurestress being raised between levels at a rate of 100 p.s.i./min. Afterreaching each of the 1,000, 2,000, 4,000 and 6,000 closure stresslevels, the system was held at 250° F. for 1.5 hours before theconductivity readings were taken. At the 6,000 p.s.i. level the cell washeld at 250° F. for 50 hours during which conductivity measurements weretaken at 10 hour intervals. The measured conductivity at the 50 hourtime was 7,351 mdft and 7084 md-ft on a second sample of the rawmaterial run at the same time.

EXAMPLE II

In this example a kaolin clay was obtained from the American CyanamidCo., Andersonville, Ga. The chemical and mineralogical composition ofthe uncalcined clay as indicated in Table I and II. This kaolin clay wascalcined a 480° C. for approximately 24 hours then crushed to less than1/8". The crushed kaolin clay was combined with 250 grade air-floatedmicrocrystalline silica provided by the Illinois Minerals Company. Thismixture contains 75 w/o clay and 25 w/o silica and hereafter will bereferred to as "blended material" in this example. The mixture wastumbled in a "V" blender for approximately 2 minutes to insure completemixing.

The blended material was next reduced in particle size using a jet millmanufactured by the Fluid Energy Processing Equipment Company ofHatfield, Pa. The average particle size of this milled material was 3.68micons as measured by the Leads and Northrup Microtrac II particle sizeanalyzer.

A 45 lb. charge of this blended material was placed in the Eirich ModelR-7 Compacting Mixer described in Example I along with 11/2 lbs. ofcornstarch binder and mixed dry for 30 seconds at the low speed setting.Water was then added to the mixer with the mixer operating at the lowspeed setting with 9 lbs. being added and mixed for 2 minutes followedby 4 additions of 2 lbs. each, one addition of 1 lb. and three additionsof 1/2 lb., with a 45 second mixing period after each addition.

The mixer was then set to the high speed mode and 10 lbs. of the blendedmaterial was slowly added to the mixer over a minute period. The mixerwas then switched back to the low speed setting and an additional 7 lbs.of the blended material was slowly added over a 2 minute period,followed by a 30 second low speed mixing. An additional 3 lbs. of theblended material was added over the following minute followed by anadditional 30 second mixing period at the low speed. Then a final 3 lbs.of the blended material was added over the following minute followed byan additional 30 second mixing period at low speed. The particles werethereafter discharged from the mixer and screened, dried and fired inthe same manner as described in Example I with the exception that thesintering temperature was 1395° C. After cooling the pellets werescreened to a 20-30 mesh size. The dry specific gravity of the materialwas 2.52. The Stim-Lab conductivity test described in connection withExample I was then performed except that the readings at the elevatedtemperature where taken at 275° C. and the maximum stress was 8000p.s.i. The test yielded a measured conductivity after 50 hours at 8,000p.s.i. and 275° F. of 3,351 md/ft.

EXAMPLE III

In this example a kaolin clay was obtained from C. E. Minerals. Thisclay was mined at their Mulcoa operation in Andersonville, Ga. Thechemical and mineralogical composition of the calcined clay is indicatedin Tables I and II. For this example the clay was calcined in a rotarykiln at 750° C. and held at that temperature for 2 hours. This clay wasnext crushed to minus 1/8 inch then milled in a similar method as thematerial in Example II. The average particle size of the milled clay inthis example was 3.01 microns as measured by the Leads and NorthrupMicrotrac II. This milled clay was then mixed with Imseil A-108 silicain a similar manner as in Example I, the ratio again being 86% clay and14% silica. Again as in the previous examples this mixture will bereferred to as "blended material" hereafter in this example.

A 45 lbs. charge of this blended material was placed in the Eirich ModelR-7 Compacting mixer described in Example I along with 11/2 lbs. ofcornstarch binder and mixed with water in the same manner as describedin Example I with the exception of adding 2 lb. additions to 17 lbs.then 1 lb. additions to 21 lbs. then a 1/4 lb. addition.

Next 13 lbs. of blended material was slowly added over a minute periodwith the mixer in the high speed setting. Next the mixer was switchedback to the low speed setting and an additional 8 lbs. of blendedmaterial was added over a period of 2 minutes after which the mixercontinued to be operated for an additional 30 seconds mixing time. Thepellets formed by this procedure were slightly wet but had a goodspherical shape. The pellets were then removed from the mixer and dried,screened (both before and after sintering) and sintered in the samesintering temperature used was 1,385° C.

The Stim-Lab conductivity test described in Example II was thenperformed on the proppant resulting in a conductivity measurement of4,459 md-ft. after 50 hours at 8,000 p.s.i. at 275° F. The dry specificgravity of the proppant was 2.59.

The low density high strength proppant particles produced in accordancewith the method of the present invention may be injected into fracturesin subsurface formation as a propping agent. In fracturing treatment aviscous fluid, often referred to as a "pad" is injected into the well atextremely high pressure to cause the formation to fail in tension andfracture to accept the fluid. The fracturing fluid may be an oil base,water base, acid, emulsion, foam or other fluid. Normally the fluidcontains several additives such as viscosity builders, drag reducers,fluid loss additives, corrosion inhibitors, cross linkers and the like.The fluid of the pad is injected until a fracture of sufficient geometryis obtained to permit the placement of the proppant pellets. Normallythe treatment is designed to provide a fracture at the well bore of atleast 21/2 times the diameter of the largest proppant pellet. Once afracture of the desired geommetry is obtained, the proppants are carriedsuspended in the fluid pad and placed in the fracture. Followingplacement of the proppant, the well is shut-in for a length of timesufficient to permit the pressure to bleed off into the formation whichin turn causes the fracture to close and exert closure stress on theproppant particles. The shutin period may vary from a few minutes toseveral days. The proppant particles of the present invention areparticularly suitable for use as propping agents in wells of depths lessthan about 14,000 feet.

What is claimed is:
 1. A method for making a low density proppantcomprising the steps of:calcining kaolin clay at a temperature belowthat which would cause the transformation of any significant portion ofsaid kaolin clay to mullite or crystobalite for a time sufficient toreduce the LOI thereof to 12% or less, when tested at 1400° C.; mixingsaid calcined kaolin clay with amorphous to microcrystalline silica;pelletizing said mixture to form pellets in a selected size range; andsintering said pellets to form proppant particles having a dry specificgravity of less than 2.70.
 2. The method of claim 1 further includingthe step of milling said calcined kaolin clay and said silica to anagglomerated particle size of 7 microns or less prior to the pelletizingstep.
 3. The method of claim 2 wherein said calcined kaolin clay andsilica are milled to an agglomerated particle size of 3 microns or less.4. The method of claims 1 wherein said calcining is performed at atemperature of less than 900° C. for a time sufficient to reduce the LOIof the kaolin clay to 2 w/o or less at 1400° C.
 5. The method of claim 1wherein said step of pelletizing includes the steps of:pelletizing aninitial amount of material which consists essentially of a mixture ofmilled, calcined kaolin clay and amorphous to microcrystalline silica bysimultaneously mixing and compacting said material while adding water ata controlled rate; and adding additional amounts of said material at acontrolled rate while continuing said simultaneous mixing and compactingto form pellets therefrom in a desired size range.
 6. The method ofclaim 5 wherein said step of pelletizing further includes dispersing anorganic binder in said material prior to adding water.
 7. The method ofclaim 5 wherein the amount of water added is between 35 w/o and 50 w/oof said initial amount of material.
 8. The method of claim 7 wherein thesaid additional amounts of said material is between 42 w/o and 70 w/o ofthe initial amount of said material.
 9. The method of claim 1 whereinsaid proppant has a conductivity at least 3000 md-ft after 50 hours at8000 p.s.i. and 275° F. in the presence of deoxygenated aqueous 2%solution of KCl as measured by the Stim-Lab Technique using sandstoneshims.
 10. The method of claim 9 wherein said conductivity is at least4000 md-ft.
 11. A method for making a low density proppant comprisingthe steps of:providing a powder mixture of between 0 and 90 w/o calcinedkaolin clay and between 10 and 100 w/o of amorphous to microcrystallinesilica, said calcined kaolin clay containing less than 2 w/o quartz,said calcined kaolin clay and silica having been milled to an averageagglomerated particle size of less than 7 microns; pelletizing saidpowder mixture by simultaneously mixing and compacting said mixturewhile adding water at a controlled rate and adding additional amounts ofsaid mixture while continuing said simultaneous mixing and compacting,to form pellets in a selected size range; and drying and sintering saidpellets to form proppant particles having a specific gravity of 2.60 andof less.
 12. The method of claim 11 wherein said calcined kaolin clay iscalcined at a temperature low enough such that substantially none ofsaid kaolin clay is transformed to mullite or crystobalite duringcalcining.